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Simulation of High Energy Density Physics

Fast Ignition Fusion

The fast ignition scheme was proposed in 1994 to rapidly heat a compressed fusion fuel by a short-duration (10ps), high-power (PW) ignition laser. It relies on fast electrons (MeV) produced by the ignition laser being transported over a 100mm distance in the coronal plasma to heat a highly compressed core region at hundred times solid density. It is key to achieve a coupling above 10% from the laser to the core. A coupling of 20% was demonstrated experimentally in 2001 with a 0.6 ps laser and a cone-inserted target. However, a recent experiment with a 10 ps laser reported much lower coupling of only a few percent. The different results could be related to different pre-plasmas formed in the cones. A commonly acknowledged factor causing the low coupling is a large divergence of the fast electrons generated in the cones.

To reduce the large divergence, we propose a magnetically assisted (MA) fast ignition scheme [1] using a cone-free target supplemented by an external 20-megagauss magnetic field to confine the fast electron motion. Such a spherically symmetric target does not suffer from an asymmetry in the target compression, as is the case for a cone-inserted target. The MA scheme was demonstrated by our integrated particle-in-cell (PIC) simulation using a two-system PIC approach recently developed by us [2]. With this PIC approach we directly obtained a laser-to-core coupling of 12% and were able to compare different schemes of fast ignition for the first time. We show that the coupling can be enhanced 7-fold with the magnetic field, which can even exceed that obtained with the cone-inserted scheme.

Comparison of FI schemesResults from a PIC simulation: Snapshots of electron densities at initial time (top row), fast-electron currents at 2ps (middle row), and resistive electric fields at 2ps (bottom row). The three columns correspond to the cone-inserted, original, and MA schemes, respectively. The ignition laser is incident along the +x direction.

Core details of MA schemeResults from a PIC simulation: Snapshots of temperatures (keV) of electrons (left column) and ions (right column). The three rows correspond to 2ps, 4ps, and 6ps from top to bottom. The core is marked with the green circle in each plot. Two counter-propagating, 2? (wavelength 0.5µm) lasers along the B-field are adopted with duration 6ps. Each laser has a power 0.5PW and energy 3kJ. At 6ps both the electrons and ions at the core are heated to 3keV with coupling from the two lasers to the core is 12%.

[1] W.-M. Wang, P. Gibbon, Z.-M. Sheng, and Y.-T. Li, Magnetically assisted fast ignition, Phys. Rev. Lett. 114, 015001 (2015). DOI: 10.1103/PhysRevLett.114.015001

[2] W.-M. Wang, P. Gibbon, Z.-M. Sheng, and Y.-T. Li, Integrated simulation approach for laser-driven fast ignition, Phys. Rev. E 91, 013101 (2015). DOI: 10.1103/PhysRevE.91.013101